The complexity and compactness of present day electronic devices requires the inclusion of thin bonding layers in various circuit configurations such as may be used to secure a semiconductor device to a heat sink or cap. Measurement of the thickness of such bonding layers is essential to assure that the devices will not only function properly but also will operate within their design parameters especially as regards the device heat transfer requirements,
Several methods presently exist for measuring the thickness of such bonding layers. An early but destructive, time-consuming method requires the cutting of the device transverse to the thickness of the buried layer and then optically measuring the thickness of the exposed buried layer. Non-destructive bonding layer thickness measurements by optical methods provide quick feedback and ensure quality. However, certain assemblies make optical measurements of the bonding layer thickness impossible.
More recently, a non-destructive method employing acoustic waves, from an acoustic microscope, has become available. Such acoustic methods can easily provide accurate thickness measurements of materials when interface reflection echoes are resolved. This is simply done by first measuring the acoustic longitudinal velocity of the bonding material and then measuring the time separation between reflected echoes.
This process provides good results only when the bonding layer is thick enough such that echo separation at both layer interfaces can be observed. Recently, Fast Fourier Transform (FFT) methods have also been employed to measure thin layer thicknesses, however these methods have yet to be successfully and robustly applied to thin bonding layers. By thin layers it is meant that the reflected acoustic signals returning from the layer are usually merged into one single echo making thickness measurements by conventional techniques very imprecise or impossible, for the reflections from both the top and the bottom of such thin bonding layers overlap another, such that the signal reflected from the top of the bonding layer cannot be differentiated from the signal reflected from the bottom of the bonding layer.
As newer, smaller semiconductor devices require bonding layers that are thinner than the threshold thickness where top and bottom echoes begin to separate, the prior art techniques are unsatisfactory and the art has been seeking a measurement technique that would provide for accurate non-destructive measuring of such thin bonding layers.